The mechanism of DNA replication in bacteriophage lambda will be studied by determining the nucleotide sequence of the replication initiation (ori) site. The replication capacity of lambda will also be exploited as a vehicle for replicating segments of DNA from drosophila. Derivatives of lambda will be constructed with restriction sites optimally located for insertion of foreign DNA. Selection techniques developed for this purpose will allow incorporation of DNA from a single band removed from a salivary gland chromosome by laser microdissection. Clones derived from a specific locus will be studied and nucleotide sequences determined.